1. Field of the Invention
The present invention relates to the field of ion exchange composite membranes. The present invention particularly describes proton exchange composite membrane, composite solution, a manufacturing method thereof, and a fuel cell comprising the same.
2. Discussion of the Related Art
Proton exchange membrane fuel cells employing hydrogen as a fuel exhibit high energy density but have several disadvantages. For example, hydrogen gas should be handled with particular care and intermediate step of reforming the alcohol into hydrogen.
Liquid-type fuel cells, on the other hand, employ liquid materials as a fuel and therefore operate at a lower temperature. Methanol is a favored fuel as it is easily obtained from natural gas or other renewable resources, it has a high specific energy density. It is liquid at operating temperatures, and most of all, the existing infrastructure for transporting petrol may easily be transformed to support methanol. A direct methanol fuel cell (DMFC) is a special form of liquid type fuel cell based on polymer electrolyte membrane fuel cell technology has received attention for a possibility of a replacement of primary and secondary batteries for potable and/or transport power source.
The DMFC can be fed with a gaseous or liquid fuel feed. As methanol is directly fed to the anode, the anode reaction consists of the oxidation of methanol, and produce carbon dioxide, electrons and protons. The overall anode reaction is given in equation (1). The proton move from the anode to the cathode via the electrolyte membrane. At the cathode, oxygen is reduced and then recombines with the protons to form water, equation (2). The overall cell reaction for the DMFC is given by equation (3).
Anode:CH3OH+H2O→CO2+6H++6e−  (1)Cathode:3/2O2+6H++6e −→3H2O  (2)Overall:CH3OH+3/2O2→CO2+2H2O  (3)Namely, water and carbon dioxide are produced by reacting methanol with oxygen.
A polymer electrolyte membrane for use in a direct methanol fuel cell should exhibit high proton conductivity, no electron conductivity, high gas impermeability, high form stability, high chemical stability, and high mechanical property, and low water and methanol permeability. Preferably, a suitable polymer electrolyte membrane also has an ion conductivity of 1˜5×10−2S/cm, a surface resistance of 0.2˜2Ω cm2, an operation temperature of 80˜120° C., and little or no methanol permeability. Due to its high ion conductivity, Nafion (from DuPont) is the most widely used as an ion exchange membrane for proton exchange membrane fuel cells. In addition to Nafion, generally used ion exchange polymer membranes include XUS (from Dow Chemical), Gore-Select (from Gore & Associates), BAM3G (from Ballad Advanced Materials of Canada), Aciplex (from Asahi of Japan), Flemion (from Asahi Glass), Product C (from Chlorine Engineering), Neospeta-F (from Tokuyama Soda), florinated cation membrane (from Hechst of Germany), and sulfonated polystyrene-block-poly (ethylene-ran-butylene)-block-polystyrene (SSEBS), which is lately being sold by Aldrich.
DMFCs, however, have a “crossover” problem, which is not a problem in PEMFC using hydrogen as a fuel, wherein methanol passes directly through the polymer membrane, from anode to cathode. The proton movement across the membrane is associated with water transport in the solvated shells of membrane. Due to the similar properties of methanol (such as its dipole moment), methanol is also transported to the cathode by electroosmotic drag. At the cathode, methanol causes a mixed potential due to its oxidation and, consequently, a decrease in cell performance. The membrane used in DMFCs were developed for PEM applications for optimising the proton conductivity; however, these membranes are not the optimal with regard to methanol blocking. As a result, there has been active research throughout the world, to develop a proton exchange membrane for DMFCs that is capable of solving the above problems.
To overcome methanol crossover, U.S. Pat. No. 6,059,943 describes a membrane added with inorganic oxide, U.S. Pat. No. 5,919,583 suggests a multi-layered membrane prepared using polymers having different ion exchangeable value, and U.S. Pat. No. 6,060,190 describes a method for inserting an electrode substrate between each layer. None of these proposals, however, has shown any advance in terms of cost or efficiency.